Typically, the high dielectric permittivity of biological media (of the order of 53 for muscle at 2.45 GHz) makes it possible to design passive sensors, notably surface wave passive sensors, operating at reduced electromagnetic wavelengths and thereby permitting likewise reduced antenna dimensions.
Currently, it has already been proposed that surface wave passive sensors of “surface acoustic wave” type, designated by the acronym SAW, be used for this type of application. The principle of this type of sensor is described hereinafter. This may according to the known art pertain notably to temperature and/or pressure sensors.
In a general manner, a complete system is composed of an interrogation unit (itself consisting of an emitter part and of a receiver part, i.e. E/R) SE/R and of a temperature and/or pressure sensor of the surface acoustic wave SAW type, as illustrated in FIG. 1. The SAW device is of resonator type, thereby making it possible to achieve structures of reduced size. The interrogation system as well as the SAW sensor are furnished respectively with antennas A1 and A2, suitably matched to the working frequency band (ISM bands 433 MHz, 868 MHz, 2.45 GHz, etc.) or to any other unoccupied frequency band of use, thereby making it possible to perform wireless interrogation of the sensor. The mode of interrogation is as follows:
The emitter of the interrogation system dispatches an interrogation signal (temporal pulse of a carrier in the ISM band, emission time slot) toward the antenna associated with the SAW resonator. Through the piezoelectric coupling effect, the incident electromagnetic wave is transformed into an acoustic wave propagating at the surface of the substrate.
If the emission signal exhibits a resonant frequency sufficiently close to the natural frequency of the SAW resonator, the latter starts resonating on passing through a charging period. Steady state oscillations are then set up at the natural resonant frequency of the SAW device. This resonant frequency is proportional to the speed of the surface wave in the resonant cavity which itself depends on the temperature and the stresses seen by the resonator.
The sensor used is based on a structure of resonator type as illustrated in FIG. 2. The resonator is composed of a transducer with inter-digitated combs (transducer at the center of the structure). The transducer consists of an alternation of electrodes, which are repeated with a certain periodicity, called the metallization period, deposited on a piezoelectric substrate (typically made of quartz). The electrodes, advantageously of aluminum, (possibly made by photolithography) have a low thickness that can go from a few hundred Angströms typically up to a micron, without this being restrictive.
The transducer furthermore comprises two ports p1 and p2 as illustrated in FIG. 2, possibly being linked to an antenna or to its radiating strands. The field lines thus created between two different polarity electrodes give rise (by virtue of the linear nature of the piezoelectric effect) to a surface acoustic wave in the zone of overlap of the electrodes for an electrical excitation at the resonant frequency of the device (given to a first approximation by the phase speed of the surface wave under the array of electrodes, divided by the electrical period of the transducer).
The sensor transmits to the antenna a signal at its resonant frequency which carries information possibly related to the pressure or temperature phenomenon. The resulting radiofrequency signal is radiated and transmitted to the receiver.
During the reception time slot, the receiver of the interrogation system detects all or part of the radiofrequency signal re-emitted (corresponding to a damped oscillation) and extracts therefrom the sought-after pressure and temperature information via suitable signal processing making it possible to identify the resonant frequency of the surface wave device.
The transducer is a bi-directional structure, that is to say the amount of energy which propagates toward the right and toward the left has the same intensity. Electrodes which have a reflector role are advantageously disposed on either side of the transducer. Each reflector partially reflects the energy emitted by the transducer. These reflectors or mirrors operate at the Bragg condition for which the mechanical period of the array corresponds to half the wavelength of the acoustic propagation.
If the number of reflectors is multiplied, a total reflection is created, thus creating a resonant cavity which is characterized by its resonant frequency. The frequency which depends on the temperature and stresses (i.e. pressure) seen by the resonator is the parameter measured by the interrogation system. The pressure and the temperature are calculated on the basis of this measurement.
According to the state of the art, pressure and temperature sensors are generally based on a differential structure using three SAW resonators, as illustrated in FIG. 3. On account of the low bandwidth allocated for ISM communications, the resonators are made on quartz on cuts for which the thermal drift of the frequency is reduced. These three resonators are enclosed in a cavity with a certain reference pressure.
The first resonator R using the customary propagation axis X is situated in a stress-free zone. The resonator T also located in a stress-free zone is tilted by a certain angle with respect to the X axis. The fact of tilting the resonator T affords the latter a different sensitivity to that of the resonator R in relation to temperature. The frequency difference between the resonators R and T consequently makes it possible to obtain information related solely to temperature independent of the state of the pressure exerted on the lower face of the device.
The resonator P using the propagation axis X like the resonator R is located in a zone where the stresses are appreciably higher than the resonators R and T so that, when an overpressure (with respect to the pressure of the cavity) is exerted on the sensor, the frequency of the resonator P varies proportionately. FIGS. 4 and 5 present two exemplary embodiments.
More precisely, in the first case illustrated in FIG. 4, the pressure of the external medium F is transmitted via a stamped metal cap Cm in contact with a zone of the quartz substrate S. The microchip bears on two projecting rectilinear metal parts pms, so that only the resonator P is subjected to significant stresses. The whole is positioned on a printed circuit Ci and connected via wire links Lf. The reference SAW designates in a common manner the three resonators R, T, P.
In the second case illustrated in FIG. 5, the pressure of the external medium is applied directly to the quartz substrate which has been thinned locally in the zone where the resonator P is situated. The other two resonators are located in a non-thinned zone bonded onto a base E and therefore less sensitive to the effects of stresses induced by the application of hydrostatic pressure to the component as a whole.
In both these cases, the resonator P uses the same direction of propagation as the resonator R (same dependence of the frequency as a function of temperature). The frequency difference between the resonators P and R consequently makes it possible to obtain information solely related to the pressure exerted independent of temperature. The base E makes it possible to position all of the resonators at the level of the printed circuit Ci.
In most industrial and automobile applications, SAW pressure and temperature sensors based on the principles described above are satisfactory in particular from the standpoint of bulk.
For certain applications, in particular for the medical sector, the size of the sensor is a determining element, in particular for implantable solutions. Even though promising trials have been performed at low frequency, it is imperative to further reduce the size of the sensor and that of the antenna associated with it.
It is in this context that the present invention proposes a novel type of passive sensor of low bulk.